Vapors condense upon a surface if the surface is cooled below the saturation temperature at a given pressure. The condensed liquid phase may accumulate on the surface as a film and/or as droplets or islands of liquid. Condensation is critical in many industrial applications, although in certain applications, it is useful to inhibit or prevent the filmwise buildup of condensing liquid on a surface by promoting droplet shedding and enhancing dropwise condensation.
Condensation of water is a crucial process in many industries, including power generation and desalination. Roughly 85% of the global installed base of electricity generation plants and 50% of desalination plants worldwide rely on steam surface condensers, a type of heat exchanger in which a plurality of tubes flowing coolant contact steam on their outside surface. Given the widespread scale if these processes, even slight improvements in cycle efficiencies will have a significant effect on global energy consumption.
One useful measure of heat transfer performance for a condenser is the heat transfer coefficient, defined as the flux per area in units of kW/m2K. Heat transfer coefficients experienced when condensing in the dropwise mode are an order of magnitude greater than those in the filmwise mode. The presence of an insulating liquid film during filmwise condensation presents a significant thermal barrier to heat transfer, whereas the departure of discrete drops during dropwise condensation exposes the condensing surface to vapor. The higher heat transfer coefficients experienced during dropwise condensation make it attractive for employing in large-scale thermal fluids applications such as steam power plants and desalination plants, as well as small-area high-heat flux applications such as electronics cooling. However, the practical implementation of dropwise condensation in power generation, desalination, and other applications has been a significant materials challenge, limited by, among other factors, durability of existing hydrophobic functionalization for metal heat transfer surfaces. While metals provide both high thermal conductivity for maximizing heat transfer and high tensile strength to minimize the need for structural supports, metals are typically wetted by water and most other thermal fluids, and, as a result, metals exhibit filmwise condensation. In order for a metal surface to exhibit desired dropwise condensation, the surface that is used for heat transfer needs to be modified. One way to achieve dropwise condensation on a metal surface where heat transfer takes place is to modify the metal surface with a hydrophobic coating.
A number of conventional techniques have been employed previously to promote dropwise condensation on surfaces, including the use of monolayer promoters such as oleic acid and stearic acid (U.S. Pat. No. 2,919,115), noble metals (U.S. Pat. No. 3,289,753 and U.S. Pat. No. 3,289,754 and U.S. Pat. No. 3,305,007), ion-implanted metal (U.S. Pat. No. 6,428,863), as well as thin films of polymers applied via sputtering or dip-coating (U.S. Pat. No. 2,923,640, U.S. Pat. No. 3,899,366, EP2143818 A1, U.S. Pat. No. 3,466,189). However, previous methods suffer from problems such as low durability and/or high cost. Moreover, most of these hydrophobic modifiers, and especially the silane-based modifiers that have been used in some conventional methods, are not robust in steam environments of industrial interest (in other words, these modifiers cannot withstand the environments in which they are used). Previous methods also do not adequately promote rapid droplet shedding because they do not sufficiently reduce the contact angle hysteresis. It is possible to have a surface with a high contact angle but also high adhesion, so even though condensation would initiate in the dropwise regime, it would ultimately progress to filmwise condensation because the drops are not able to shed easily.
Furthermore, where the condensing liquids are hydrocarbons or other low-surface tension liquids, the problem of film-wise condensation is exacerbated. Current surfaces designed for dropwise condensation of water do not promote dropwise condensation of low-surface tension hydrocarbon liquids such as n-alkanes (e.g., octane, hexane, heptane, pentane, butane) and refrigerants (e.g., fluorocarbons, chlorofluorocarbons, hydrochlorofluorocarbons) and cryogenic liquids (e.g., LNG, C2, N2, CO2, methane, propane).
Some conventional methods have used nanotextured surface to improve condensation heat transfer, however, these methods also rely on silane or thiol modifiers to modify the wettability of a nanotextured surface from superhydrophilic to superhydrophobic, and thus these nanotextured surfaces are subject to the same robustness concerns discussed above. Additionally, because the thermal conductivities of polymeric materials are typically orders of magnitude smaller than those of a typical metal substrate, the thickness of the polymer modifier is extremely important. Hence, there is currently a need for an ultra-thin robust hydrophobic modifier that may be applied over a metal surface to enhance heat transfer.
There is a need for methods and articles/devices for improved heat transfer and/or dropwise condensation of low-surface tension liquids, including hydrocarbon liquids.